# Candidate Water Vapor Lines to Locate the H$_{2}$O Snowline through High-Dispersion Spectroscopic Observations II. The Case of a Herbig Ae Star

## TL;DR

This study identifies specific water vapor emission lines across various wavelengths that can be used to locate the H$_{2}$O snowline in Herbig Ae star disks, aiding understanding of planet formation and water distribution.

## Contribution

It calculates candidate water emission lines with small Einstein A coefficients suitable for locating the H$_{2}$O snowline in Herbig Ae disks, expanding observational strategies.

## Key findings

- Water lines with high upper state energies are promising for snowline detection.
- Fluxes of candidate lines increase at shorter wavelengths.
- Detection prospects are better for Herbig Ae disks than T Tauri disks.

## Abstract

Observationally measuring the location of the H$_{2}$O snowline is crucial for understanding the planetesimal and planet formation processes, and the origin of water on Earth. In disks around Herbig Ae stars ($T_{\mathrm{*}}\sim$ 10,000K, $M_{\mathrm{*}}\gtrsim$ 2.5$M_{\bigodot}$), the position of the H$_{2}$O snowline is further from the central star compared with that around cooler, and less massive T Tauri stars. Thus, the H$_{2}$O emission line fluxes from the region within the H$_{2}$O snowline are expected to be stronger. In this paper, we calculate the chemical composition of a Herbig Ae disk using chemical kinetics. Next, we calculate the H$_{2}$O emission line profiles, and investigate the properties of candidate water lines across a wide range of wavelengths (from mid-infrared to sub-millimeter) that can locate the position of the H$_{2}$O snowline. Those line identified have small Einstein $A$ coefficients ($\sim 10^{-6} -10^{-3}$ s$^{-1}$) and relatively high upper state energies ($\sim$ 1000K). The total fluxes tend to increase with decreasing wavelengths. We investigate the possibility of future observations (e.g., ALMA, SPICA/SMI-HRS) to locate the position of the H$_{2}$O snowline. Since the fluxes of those identified lines from Herbig Ae disks are stronger than those from T Tauri disks, the possibility of a successful detection is expected to increase for a Herbig Ae disk.